In the following descriptions, radian is used as a unit for phases. In recent years, there has been developed a polarization imaging camera that simultaneously obtains information on polarization in a plurality of directions. As a result, proposals are made for a polarization imaging apparatus for visualizing a polarized-light distribution of an object (Patent Literature 1) and a polarization microscope for visualizing a polarized-light distribution of an object (Patent Literature 2). Possible applications of polarization imaging are wide-ranging. Examples of such possible applications encompass (i) measurement of a structure of and/or a distortion of a material such as film or glass, which is used as a window material, a shop window, a display or the like, (ii) measurement of a film pressure of and a distortion of a thin film of a solar cell. Examples of possible applications of a polarization microscope encompass (a) characteristic evaluation of a crystal structure or a molecular structure, (b) identification of a rock forming mineral, (c) visualization of an internal structure of a living body (cell) without staining, and (d) visualization of a distribution of protein, collagen, or the like in a living cell.
A polarization imaging technique has been improved according to industrial requirements. However, the above-mentioned polarization imaging techniques cannot realize imaging of an instantaneous three-dimensional structure because of, for example, the following reasons. Specifically, a polarization microscope needs to raise a magnifying power of an objective lens, for observation of an object in a microscopic area. Accordingly, a photographable area extending in a depth direction becomes extremely narrow. Therefore, (i) it takes long to complete a product inspection because images should be taken multiple times, (ii) it is very difficult to observe how chemical structures change over time at different depth positions, and (iii) it is very difficult to observe, via a motion picture, how metabolite or the like behaves that is three-dimensionally spread in a living body (cell).
In order to solve the above-mentioned problems, some digital holography techniques for polarization imaging have been recently proposed. For example, Non-Patent Literature 1 discloses a technique for polarization imaging in which off-axis type digital holography is employed. According to such off-axis type digital holography, object light and reference light enter an image pickup element at different angles. It is therefore possible to obtain only a desired object image while a hologram is being reconstructed. This is because zero-order diffracted light, a conjugate image (minus first-order diffracted light), and an object image (first-order diffracted light) do not overlap each other. According to the configuration disclosed in Non-Patent Literature 1, it is possible to realize imaging of (i) an instantaneous three-dimensional structure and (ii) an instantaneous polarized-light distribution of an object. That is, it is possible to concurrently obtain distributions of polarized light of an object image at respective different positions in a depth direction.
FIG. 53 is a view illustrating a structure of a configuration of a conventional polarization imaging apparatus described in Non-patent Literature 1. FIG. 54 is a view illustrating a relation of reference light R1 and R2 and object light that enter an image pickup device provided in the polarization imaging apparatus shown in FIG. 53. This conventional polarization imaging apparatus (i) causes the reference light R1 and R2 to enter at different angles (for convenience, expressed in θ1 and θ2) from different directions with respect to the object light, respectively, which reference light R1 and R2 has respective components P1 and P2 in different polarization directions, and (ii) obtains a single-sheet interference figure (hologram).
FIG. 55 is a view illustrating a procedure in which an image is reconstructed from a hologram that is recorded by the polarization imaging apparatus. An obtained interference figure is subject to Fourier transform, and then a spatial spectrum distribution is obtained by calculation. Pieces of spatial spectrum information on an object in the respective polarization directions P1 and P2 are extracted. Subsequently, the pieces of information of the object in the respective polarization directions are subjected to (i) phase corrections, only by amounts relating to the respective angles θ1 and θ2, (ii) reverse Fourier transform, and (iii) image reconstruction by a diffraction calculation. After the image reconstruction, polarization imaging is performed while using complex amplitude distributions of the object in the respective polarization directions P1 and P2.
Non-Patent Literature 2 discloses a polarization imaging technique while using in-line type or on-axis type digital holography. According to the in-line type digital holography, object light and reference light enter an imaging element at identical angles. While a hologram is being reconstructed, (i) a zero-order diffracted light and a conjugate image (minus first-order diffracted light) which are noise components and (ii) an object image (first-order diffracted light) overlap each other. In order to obtain only a desired object image, it is therefore necessary to (i) sequentially photograph a plurality of holograms having respective different phases, respective different optical path lengths, or the like and (ii) make a calculation for extracting only the object image by use of a method such as a phase shift method or an optical-path-length shift method.